If a line has slope m = 0, what type of line is it?

• A. Horizontal line
• B. Vertical line
• C. Diagonal line with positive slope
• D. Undefined slope line

Answer: (A)

Slope describes how steep a line is, measured as the rise over the run. If the slope is zero, the line does not rise or fall as you move along it, so it runs perfectly flat. In other words, the vertical change is zero while you can still move horizontally, which means the line is parallel to the x-axis and has the equation y = b for some constant b.

This is different from a vertical line, where the horizontal change is zero, giving an undefined slope; and different from a diagonal line with positive slope, which rises as you move to the right, giving a positive slope. So a line with slope zero is a horizontal line.